A variable resistor element is an element capable of electrically switching at least two resistance values such as a high resistance state and a low resistance state. Since the variable resistor element is generally configured from an insulating material, an initial step (hereinafter referred to as forming) of forming a conductive path in the variable resistor element is required to enable the resistance state to be electrically switched. Specifically, the forming is carried out by applying a voltage between an upper electrode (bit line) and a lower electrode (word line) of a selected variable resistor element. Although only the current of about 10 nA is flowed even if the forming voltage (e.g., 5V) is applied since the variable resistor element before forming is in an insulating state, the variable resistor element becomes lower resistance and the current of a few μA is flowed when forming is carried out. When made to lower resistance, the variable resistor element is able to reset (higher resistance state) or set (lower resistance state), so that data can be stored. The forming process is difficult to carry out on a plurality of bits (a plurality of variable resistor elements) collectively, and thus the process tends to take time.